Process for forming passivated film

ABSTRACT

A process for a passivated film which is far reduced in the amount of gas discharge and can desorb an adsorbed gas more readily, which process comprises heating a stainless member with a surface roughness, Rmax, of 1.0 μm or less in an atmosphere of a mixture comprising oxygen gas and an inert gas and having a dew point of -95° C. or below, an impurity concentration of 10 ppb or less and an oxygen content 5 ppm 25 vol % at 300° to 420° C.

TECHNOLOGICAL FIELD

The present invention relates to a process for forming passivated film,and in particular relates to a process for forming passivated film whichis capable of forming a passivated film which has an extremely smallrelease of moisture, and which is capable of conducting the desorptionof adhering moisture in an extremely short period of time.

BACKGROUND ART

In recent years, technologies which realize ultrahigh-grade vacuums, andtechnologies which create ultraclean low pressure atmospheres by meansof the inflow of small amounts of specified gasses into a vacuumchamber, have become extremely important.

Such technologies are widely used in research into the characteristicsof materials, the formation of various types of thin films, and theproduction of semiconductor devices, and therefore higher and higherdegrees of vacuum are being realized; however, furthermore, therealization of a low pressure atmosphere in which contamination byimpurity elements or impurity molecules is limited to an extreme degreehas been greatly desired.

For example, to use the example of semiconductor devices, as a result ofthe increase in the degree of integration of integrated circuits, thedimensions of the unit elements have become smaller each year, and assemiconductor devices having dimensions such that the spaces betweenelements have gone from a level of 1 micrometer to a submicrometerlevel, and further to a level less than 0.5 micrometers, have come intocommon use, research and development in this area has been conducted ona large scale.

The production of this type of semiconductor device is accomplished bymeans of the repetition of a process in which a thin film is formed, anda process in which this thin film is subjected to etching in a specifiedcircuit pattern. It is common for this type of process to be conductedin an ultrahigh vacuum state or in a low pressure atmosphere in which aspecified gas is introduced, by means of placing a silicon wafer in avacuum chamber. In such processes, if contamination with impurities ispresent, for example, problems will be caused in that the quality of thethin film will be reduced, and sufficient accuracy will not beobtainable in the very detailed treating. This the reason why anultrahigh vacuum and an ultraclean low pressure atmosphere have beendesired.

One of the greatest obstacles to the realization of an ultrahigh vacuumor an ultraclean low pressure atmosphere has, up until now, been gaswhich was discharged from the stainless steel surface which is widelyused in the chamber and in the gas pipes. In particular, it has beendetermined that the greatest source of contamination is from moisturewhich adsorbs to the surface and desorbs in a vacuum or in a lowpressure atmosphere.

FIG. 5 is a graph showing the relationship between gas contamination andthe total leak amount (the sum of the discharge gas amount from thesurfaces of the pipe system and the interior of the reaction chamber andexternal leaks) of a system in a conventional device in which a gas pipesystem and a reaction chamber are combined. The plurality of lines inthe drawing indicate cases in which the flow amount of the gas ischanged to various values as a parameter.

Semiconductor processing is exhibiting a tendency to reduce the gas flowamounts to a greater and greater extent in order to realize highlyaccurate processing; for example, it has now become common to use flowamounts of 10 cc/min or less.

Assuming a flow amount of 10 cc/min, if, as in presently widely useddevices, a system total leakage on the order of 10⁻³⁻¹⁰ ⁻⁶ Torr.l/sec ispresent, the gas purity will be 10 ppm-1%, which is well outside highlyclean processing ranges.

The present inventors have invented a ultrahigh-purity gas supply systemwhich has succeeded in reducing the leakage amount from the exterior ofthe system to a level of less than 1×10⁻¹¹ Torr.l/sec, which is belowthe detecting threshold of present detectors.

However, as a result of leaks from the interior of the system, that isto say, as a result of gas components discharged from theabove-described stainless steel surfaces, it has been impossible toreduce the impurity concentration of the low pressure atmosphere. Theminimum value of the surface discharged gas amount obtained by means ofthe surface treating available in the present ultrahigh vacuumtechnology is 1×10⁻¹¹ Torr.l/sec.cm² in the case of stainless steel, andeven if the surface area which is exposed in the interior of the chamberis estimated at a value which is as small as possible, for example, 1m², a total leakage amount of 1×10⁻⁷ Torr.l/sec results, and a gashaving a purity of only approximately 1 part per million with respect toa gas flow amount of 10 cc/min can be obtained. If the gas flow amountis further reduced, it is of course obvious that the purity will furtherdecline.

In order to reduce the degassing component from the inner surfaces ofthe chamber to a level of approximately 1×10⁻¹¹ Torr.l/sec, which isequal to the external leakage amount of the total system, it isnecessary to reduce degassing from the stainless steel surfaces to lessthan 1×10⁻¹⁵ Torr.l/sec.cm² ; for this reason, a stainless steel surfacetreating technique which can reduce the gas discharge amount has beengreatly desired.

On the other hand, in semiconductor production processes, a greatvariety of gasses are in use, from relatively stable common gasses (O₂,N₂, Ar, H₂, He), to rare gasses having great reactivity, corrosivity,and toxicity. In particular, if moisture is present in the atmosphere ina special gas, this may hydrolyze, producing hydrochloric acid orhydrofluoric acid, and boron trichloride (BCl₃) and boron trifluoride(BF₃) and the like, which exhibit strong corrosivity, will be present.Normally, stainless steel is used as a material for pipes and chambershandling such gasses, in view of its reactivity, resistance tocorrosivity, high strength, ease of secondary working, ease of welding,and ease of polishing the inner surfaces thereof.

However, although stainless steel has superior resistance to corrosionin an atmosphere of dry gas, it corrodes easily in an atmosphere of achlorine or fluorine system gas in which moisture is present. As aresult, it is necessary to conduct corrosion resistant treating afterthe surface polishing of the stainless steel. Among such treatings,coating of a metal which has superior resistance to corrosion, such asNi--W--P, onto the stainless steel is known; however, in this method,not merely are cracks and pin holes and the like easily caused, but asthis is a method which uses wet plating, there are problems in that theamount of moisture adsorption or the residual solution component on theinner surface is large, and the like.

An example of another method is corrosion resistant treating by means ofpassivation treating which creates a thin oxide film on a metal surface.If sufficient oxidizer is present in a liquid, stainless steel can bepassivated simply by means of immersion, so that normally, passivationtreating is conducted by means of immersion in a nitric acid solution atnormal temperatures.

However, this method is also a wet method, so that moisture and residualplating solution are present in large amounts on the pipes and on theinner surface of the chamber. In particular, in the case in which themoisture discharges chlorine system and fluorine system gasses, severedamage is caused to the stainless steel.

Accordingly, the construction of a chamber or gas pipe system by meansof stainless steel having formed thereon a passivated film which doesnot receive damage even from corrosive gasses and which has lowocclusion and adsorption of moisture is extremely important in very highvacuum technologies and in semiconductor processing; however,previously, this type of technology has not been available.

The present applicants have, on Feb. 4, 1988, filed a patent applicationJapanese Patent number 2459/88 for a stainless steel member, wherein thepercentage of Ni atoms in an outer layer of an oxide film formed on astainless steel member surface which was subjected to electrolyticpolishing treating is less than 2%, and the percentage of Cr atoms in aninner layer thereof is more than 30%, and the thickness of this oxidefilm is within a range of 10-50 nm, and for a stainless steel member andproduction method, wherein heat treating is conducted in an atmosphereof an oxide gas having a moisture dew point of from -10° C.-<-105° C.(Applicant: Tadahiro Ohmi)

This stainless steel member enables the simple conducting of desorptionof the moisture by means of conducting appropriate baking, even ifmoisture adheres or is adsorbed, and this stainless steel member alsohas a small gas discharge amount from the member itself.

However, as the effects on the characteristics of the semiconductorprocessing and the like which are caused by the purity of the treatinggas have become clearer, and as it has come to be understood that as thepurity increases, devices with greater abilities can be obtained, thedevelopment of a stainless steel member which has an even smaller gasdischarge amount, and furthermore, is able to more easily control thedischarge of adsorbed gasses, has been strongly desired.

DISCLOSURE OF THE INVENTION

The process for forming a passivated film of the present inventors,which solves the above problems, forms a passivated film by means of theheating of a stainless steel member, having a surface roughness valueRmax which is less than 1.0 micrometers, to a temperature of 300°-420°C. in a mixed gas atmosphere in which an oxygen gas containing oxygen ata rate of 5 ppm-25 vol % and an inert gas are mixed, wherein the dewpoint is less than -95° C., and the impurity concentration is less than10 parts per billion.

Function

The present inventors have conducted research into the development of astainless steel member which further reduces the discharge of moisture.As a result, they have discovered that if the formation of a passivatedfilm is conducted under specified conditions, a passivated filmcomprising an amorphous oxide can be formed, and furthermore, when thispassivated film was tested, it was discovered that the film possessesminuteness and that the resistance to gas discharge thereof representsan improvement over that of the stainless steel member for which apatent was previously sought.

The present invention is based on the above discoveries; hereinbelow, itwill be explained in detail.

In the present invention, the surface roughness Rmax of the stainlesssteel member is set to a level of below 1.0 micrometers. If the value ofRmax exceeds 1.0 micrometers, the oxide film which is formed is lackingin minuteness, so that the expected increase in gas discharge resistancecannot be realized. In the range of Rmax below 1.0 micrometers, therange of 0.1 micrometers-0.5 micrometers is further preferable. If themaximum value of the difference in height between convexities andconcavities in a circular region having a radius of 0.5 micrometers atany freely selected location is set to a value less than 1 micrometer,minuteness is further improved, and the formation of a passivated layerhaving a small gas discharge is possible. Furthermore, if the adjustmentof the surface roughness is accomplished, for example, by electrolyticpolishing, even if a deformed layer is present, this deformed layer willbe eliminated, and the adsorption of gas to this deformed layer can beprevented, so that this is preferable.

On the other hand, in the present invention, the dew point of theatmospheric gas is set lower than -95° C. By means of the limitation ofthe dew point to a temperature which is less than -95° C., as statedhereinbelow, the restriction of the impurity concentration and theheating temperature are aided, and by means of the minuteness, itbecomes possible to form an amorphous passivated film which has superiorresistance to gas discharge. If the dew point exceeds a temperature of-95° C., the passivated film will not have sufficient minuteness and theresistance to gas discharge will be poor. The fact that if the dew pointexceeds the temperature of -95° C., the minuteness of the passivatedfilm will be insufficient, and the resistance to gas discharge willdeteriorate, was discovered by the present inventors. A temperature ofless than -110° C. is still further preferable.

On the other hand, in the present invention, heat treating is conductedin an atmosphere of a mixed gas in which an oxygen gas containing oxygenat a rate of 5 ppm-20 vol % and an inert gas are mixed.

In the present invention, it is possible to form an amorphous passivatedfilm which has sufficient minuteness even with an oxygen amount of 5ppm-20 vol % by means of the controlling of the dew point and ofimpurities. However, at levels of less than 5 ppm, the amount of oxygenis insufficient, and the formation of a satisfactory oxide film isdifficult. Furthermore, if 20 vol % is exceeded, the resistance to gasdischarge worsens.

On the other hand, the impurity concentration in the atmospheric gas isset to a total level of less than 10 ppb. A level of 5 ppb or less ispreferable, while a level of 1 ppb or less is still further preferable.If a level of 10 ppb is exceeded, the passivated film will not possesssufficient minuteness even if the other conditions are within the rangesof the present invention.

The heating for the purpose of passivated film formation is conducted ata temperature within a range of 300°-420° C. At temperatures less than300° C., the temperature is too low and an oxide film possessingsufficient vertical density cannot be formed. When the temperatureexceeds 420° C., a crystalline passivated film is formed. Accordingly,the heat temperature is within a range of 300°-420° C.

The heating period varies with the the heating temperature, however, aperiod of more than 30 minutes is preferable.

The passivated film formed according to the above method normally has athickness of 10-20 nm, and comprises an amorphous oxide which is rich inCr atoms on the side of the member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram showing an example of a device for thepurpose of conducting passivating treating.

FIG. 2 is a conceptual diagram showing a testing device for resistanceto gas discharge.

FIG. 3 is a graph showing resistance to gas discharge.

FIG. 4(a) and (b) are is a scanning electron micrographs of passivatedfilm showing a crystalline structure of the film.

FIG. 5 is a graph showing the relationship between the leakage amount ofa conventional gas supply pipe system and impurity concentrations.

PREFERRED FORM FOR THE EXECUTION OF THE INVENTION

The inner surface of a SUS316L stainless steel pipe having an outerdiameter of 12.7 mm, a thickness of 1 mm, and a length of 2 m wassubjected to electrolytic polishing using an aqueous solution of H₂ SO₄-H₃ PO₄, and the surface roughness thereof was brought within a range of0.1-1.0 micrometers. Furthermore, the largest value of the difference inheight between concave portions and convex portions within a 5micrometer radius was set to a value of less than 1.0 micrometers.

The stainless steel pipe was placed in the device shown in FIG. 1, andthe formation of a passivated film was conducted. Reference numeral 101indicates the stainless pipe in FIG. 1. Reference numeral 105 indicatesa header; a plurality of gas input ports 110 are formed on header 105. Ataper is provided on the outer circumference of the lead end of inputports 110, and it is possible to support the stainless steel pipe 101 inthese tapered portions.

Reference numeral 103 indicates an inert gas source (in the presentexample, an Ar gas source), and reference numeral 104 indicates anoxygen source, and the gasses from inert gas source 103 and oxygen gassource 104 are mixed through the medium of mass flow controllers 105 and106, and this flows into the interior of the stainless steel pipe 101from input port 110. By means of this device, the impurity concentrationof the gas which is supplied to the interior of the stainless steel pipecan be reduced to a level of ones of ppb or less.

Reference numeral 121 and 122 indicate inert gas sources which supplyinert gas to the interior of furnace 130, prevent the oxidation of theouter surface of stainless steel pipe 101, and furthermore, preventburning.

Reference numeral 102 indicates a heater.

Using the device shown in FIG. 1, a passivated film is formed accordingto the following procedure.

That is to say, using an inert gas (for example, Ar or He) having animpurity (moisture, hydrocarbons) concentration of less than 10 ppb, theinner surface of a stainless steel pipe 101 is purged, and aftermoisture has been sufficiently removed, heating to a temperature ofapproximately 150° C. is conducted and a further purge is conducted, andwater molecules adsorbing to the inner surface of this stainless steelpipe 101 are desorbed essentially completely. Next, a mixed gas ofoxygen and an inert gas (Ar gas) was introduced and heating wasconducted, in accordance with the various conditions shown in Table 1,and a passivated film was formed.

The following test were conducted on stainless steel pipes possessingpassivated films formed by means of the above process.

Gas Discharge

Resistance to gas discharge was tested by means of the mechanismindicated in FIG. 2. That is to say, a mixed gas of oxygen gas and Argas which had been passed through a gas purification device 401 waspassed through a stainless steel pipe 402 which was to be tested at aflow amount of 1.2 l/min, and the amount of moisture contained in thegas was measured by means of a APIMS (Atmospheric Pressure IonizationMass Spectrometer) or low temperature optical dew point instrument 403.The results thereof are shown in FIG. 3.

Crystallization

Crystallization was tested by means of a scanning electron microscope orthe like.

The results of the above-described tests are shown in Table 1, FIG. 3,FIG. 4(a), and FIG. 4(b).

    __________________________________________________________________________              Dew-Point                                                                            Impurities    Heating     Resistance                                   Temperature                                                                          Present in                                                                             O.sub.2                                                                            Tempature                                                                            Film to Gas                                    No.                                                                              °C.                                                                           Mixed Gas (ppb)                                                                        Content                                                                            °C.                                                                           Quality                                                                            Discharge                          __________________________________________________________________________    Preferred                                                                            1  <-100  <10      20%  415    Amorph-                                                                            O                                  Embodiment                            ous                                     Preferred                                                                            2  <-100  <10       5%  415    Amorph-                                                                            O                                  Embodiment                            ous                                     Comparative                                                                          3  <-100  <10      30%  415    Amorph-                                                                            X                                  Example                               ous                                     Comparative                                                                          4    -90  >10      20%  415    Amorph-                                                                            X                                  Example                               ous                                     Comparative                                                                          5    -50  <10      20%  415    Amorph-                                                                            X                                  Example                               ous                                     Preferred                                                                            6  <-100  <10      20%  350    Amorph-                                                                            O                                  Embodiment                            ous                                     Comparative                                                                          7  <-100  <10      20%  550    Crystal-                                                                           X                                  Example                               line                                    Comparative                                                                          8  <-100  <10      20%  250    Amorph-                                                                            X                                  Example                               ous                                     Preferred                                                                            9  <-110  <10      20%  415    Amorph-                                                                            OO                                 Embodiment                            ous                                     Preferred                                                                            10 <-100  <10      6 ppm                                                                              415    Amorph-                                                                            O                                  Embodiment                            ous                                     Comparative                                                                          11 . . .  . . .    . . .                                                                              . . .  . . .                                                                              X                                  Example                                                                       __________________________________________________________________________     Resistance to Gas Discharge:                                                  OO = very good,                                                               O = good,                                                                     X = poor                                                                 

As shown in Table 1, preferred embodiments 1, 2, 6, 9, and 10, the dewpoint temperature, impurities present in mixed gas, oxygen content, andheating temperature of which are all within the prescribed ranges of thepresent invention, were all superior in resistance to gas discharge. Inparticular, preferred embodiment 9, the dew point of which was less than-110 ° C., had even more superior resistance to gas discharge.

In preferred embodiment 9, passivating treating was conducted at atemperature of 415° C. and as shown in the scanning electron micrographof FIG. 4(a), the passivated film was an amorphous film possessingminuteness.

In contrast with the above-described preferred embodiments, comparativeexample 3 has an oxygen content which is greater than the prescribedrange of the present invention, comparative example 4 has a dew pointtemperature higher than the prescribed range of the present invention,and furthermore, has an impurity concentration in the mixed gas which ishigher than the prescribed range of the present invention, comparativeexample 5 has a dew point temperature which is higher than theprescribed range of the present invention, comparative example 7 has aheat treating temperature which is too high, and comparative example 8has a heat treating temperature which is too low, so that all of thesecomparative examples had poor resistance to gas discharge.

In comparative example 7, passivating treating was conducted at atemperature of 550° C., and as shown in the scanning electron micrographof FIG. 4(b), this passivated film, in which the particle boundaries canbe clearly recognized, has a large crystalline structure. In the case ofcomparative example 11, comparative example 11 is in anas-electropolished state, that is to say, a state in which passivationtreating has not been conducted, so that the resistance to gas dischargethereof was not good.

Possibilities for Use in Industry

In accordance with the present invention, it is possible to form apassivated film possessing superior resistance to gas discharge.

We claim:
 1. A method of forming a passivated film on a stainless steelmember having a surface roughness, Rmax, of less than 1.0 micrometers,said method comprising heating said stainless steel member to atemperature within a temperature range of between 300° C. and 420° C. ina mixed gas atmosphere, said mixed gas atmosphere comprising an inertgas and oxygen and having an oxygen concentration within an oxygenconcentration range of between 5 ppm and 20 volume percent, a dewpointtemperature of less than -95° C., and an impurity concentration of lessthan 10 ppb, the stainless steel member being heated for a timesufficient to form the passivated film.
 2. The method of claim 1 whereinsaid dewpoint temperature is less than -110° C.
 3. The method of claim 1wherein said impurity concentration is less than 5 ppb.
 4. The method ofclaim 1 wherein the impurity concentration is less than 1 ppb.